Photographic emulsions are sensitive to light because of the presence of silver halide grains with different sizes, structures or compositions. The size of the silver halide grains is directly related to the sensitivity of the photographic emulsion obtained. In fact, the larger the silver halide grains making up the photographic emulsions, the more sensitive are the latter.
However, emulsions with large grains have a higher granularity than fine-grain emulsions. There is therefore a predictable relationship between photographic sensitivity and granularity. It has been possible to determine empirically that whenever the sensitivity of an emulsion is increased twofold, an increase in granularity of 7 to 10 units is obtained.
Polydisperse emulsions which have a normal grain size distribution curve offer a good compromise, which enables sensitive emulsions to be obtained whilst retaining low granularity. Such emulsions can be obtained directly by precipitating silver halide grains, or by mixing monodisperse emulsions of different mean sizes. This technique of mixing monodisperse emulsions enables emulsions to be obtained with a particularly reproducible polydispersity.
Photographic products comprising at least one blend of photographic emulsions with different sensitivities are described in the prior art. For example, U.S. Pat. No. 4,689,292 describes such a light-sensitive photographic product having high sensitivity and good covering power.
The products described in the aforesaid patent comprise at least one silver halide emulsion where the size distribution curve for the grains making up the emulsion has at least two peaks separated by 0.1 to 0.3 .mu.m, that is to say the photographic emulsion consists of at least two populations of silver halide grains having mean grain sizes which differ by at least 0.1 .mu.m and no more than 0.3 .mu.m. These emulsions are spectrally sensitized by particular spectral dyes affording sensitization of the orthochromatic type. This emulsion may be obtained from polydisperse and/or monodisperse emulsions. This emulsion may be utilized in a single layer or in different superimposed layers. In the examples illustrating U.S. Pat. No. 4,689,292, the emulsions used are obtained from blends of polydisperse and/or monodisperse emulsions with a core and a shell, referred to as "core-shell emulsions", containing at least 20% iodide in the core. The emulsions obtained have improved speed and better resistance to pressure, and the photographic products comprising this blend of photographic emulsions are preferably used in medical radiography.
European Patent Application 83239 describes a color photographic product formed by several layers of light-sensitive photographic emulsions. These sensitive layers are such that at least 80% of the total number of silver halide grains in the product have a mean grain size larger than 0.8 .mu.m or smaller than 0.65 .mu.m. These silver halide grains of different sizes may be situated in the same photographic layer or in different layers. The emulsions used are polydisperse emulsions or blends of monodisperse emulsions with different mean grain diameters. In the description in this European patent application, it is mentioned that the composition of the silver halide grains constituting the blends does not represent a limiting factor in the invention. However, the emulsions of the blend which are preferred and described in the examples have identical compositions and are formed by silver bromoiodide grains containing 4% iodide. In the examples described in Tables 1 and 3, it can be seen clearly that the photographic emulsions, in which at least 80% of the total number of silver halide grains have a mean grain size larger than 0.8 .mu.m or smaller than 0.65 .mu.m, have improved sharpness and granularity compared with a polydisperse emulsion comprising a distribution of grain sizes outside the range claimed. However, the speed of each of the blends of emulsions of the invention lies between the speed of the most sensitive emulsion and the sensitivity of the least sensitive emulsion.
European Patent Application 63962 describes a color photographic product comprising one or more layers of silver halide emulsions formed by at least two monodisperse emulsions having mean grain sizes of between 0.2 and 3.0 .mu.m. The size distribution curve for the silver halide grains has two peaks separated by at least 0.3 .mu.m. The silver halide photographic product obtained is a high-speed photographic product having improved granularity. The mean size of the grains in the monodisperse emulsions used is preferably between 0.5 and 1.4 .mu.m. The grains constituting the monodisperse emulsion or emulsions in the patent EP 63962 may consist of one or more silver halides. However, it is preferred to use silver bromoiodide or bromochloride grains in which the silver bromide is the main constituent of the silver halide grains. In the examples, the emulsions used are silver bromoiodide emulsions containing 2% silver iodide.
The monodisperse emulsions described above may be used in different layers or in a blend in one and the same layer. In Table 1 in EP 63962, it can be seen clearly that the granularity of a blend of emulsions with tabular monodisperse bromoiodide grains is improved compared with the granularity of the control emulsion, which in this case is a polydisperse bromoiodide emulsion with tabular grains.
U.S. Pat. No. 3,989,527 describes a photographic product comprising at least one radiation-sensitive layer. This radiation-sensitive layer contains silver halide grains which are surface-sensitized by a spectral sensitizer. These grains are intimately mixed with silver halide grains with a mean grain diameter of between 0.15 and 0.5 .mu.m and which are not spectrally sensitized. These grains which are not spectrally sensitized enable the exposure radiation to be reflected. These grains, known as "reflecting grains", represent at least 1% by weight of the total silver halide grains. The reflecting grains preferably form a monodisperse population. Their size will be chosen as a function of the wavelength of the exposure radiation. These emulsions, consisting of spectrally sensitized silver halide grains and reflecting grains, offer an increase in speed without degradation of the granularity. In the examples in the patent, the reflecting grains are monodisperse pure bromide grains (0.48 .mu.m) with a cubic structure. These grains, which are neither chemically nor spectrally sensitized, are insensitive to the exposure radiation of the product.
U.S. Pat. No. 4,865,964 describes a photographic product comprising a blend of bromide or bromoiodide emulsions having tabular grains with a high aspect ratio and bromide or bromoiodide emulsions having tabular grains with a low aspect ratio. This blend of emulsions makes it possible to obtain an advantage with regard to speed and granularity when the photographic speeds of each of the emulsions making up the blend are relatively close together.
As is shown by the prior art described above, the blends of emulsions are often used for improving the sensitometric properties of photographic emulsions. Indeed, the granularity of a blend of emulsions can be reduced in a predictable manner by substituting, for some of the coarse grains making up the blend, grains of smaller sizes, since the granularity of a photographic image is directly related to the size of the silver halide grains in the emulsion. In such case, the speed of the blend is between the speed of the slow emulsion and the speed of the fast emulsion.
In addition, it is known that the speed of a photographic emulsion can be increased by increasing the size of the silver halide grains, which necessarily increases the granularity.
In all the patents described above, the blends of emulsions are obtained from monodisperse silver halide emulsions with different sensitivities, without any particular conditions with regard to the silver halide composition of these emulsions.